Catalytic Converter

The catalytic converter is a muffler like container built into the exhaust system to aid in the reduction of exhaust emissions. The catalyst element consists of individual pellets or a honeycomb monolithic substrate coated with a noble metal such as platinum, palladium, rhodium or a combination. When the exhaust gases come into contact with the catalyst, a chemical reaction occurs which will reduce the pollutants into harmless substances like water and carbon dioxide.

Fig. Fig. 1: Oxidizing catalytic converter system

Fig. Fig. 2: 3-way catalytic converter system

There are essentially two types of catalytic converters: an oxidizing type is used on some models. It requires the addition of oxygen to spur the catalyst into reducing the engine's HC and CO emissions into H
2
O and CO
2
. Because of this need for oxygen, the Air Injection system is used with all these models.

The oxidizing catalytic converter, while effectively reducing HC and CO emissions, does little, if anything in the way of reducing NOx emissions. For this reason, the three way catalytic converter was developed.

The three way converter, unlike the oxidizing type, is capable of reducing HC, CO and NOx emissions; all at the same time. In theory, it seems impossible to reduce all three pollutants in one system since the reduction of HC and CO requires the addition of oxygen, while the reduction of NOx calls for the removal of oxygen. In actuality, the three way system really can reduce all three pollutants, but only if the amount of oxygen in the exhaust system is precisely controlled.

The catalytic converter and related parts should be inspected periodically for mechanical damage. The air injection system should be checked at the same time, and maintained in proper working at all times.

PRECAUTIONS

Use only unleaded fuel.

Avoid prolonged idling. The engine should run on longer than 20 min. at curb idle and no longer than 10 min. at fast idle.

Do not disconnect any of the spark plug leads while the engine is running.

Make engine compression checks as quickly as possible.

TESTING

Testing the catalytic converter operation in the field is a difficult problem. The most reliable test is a 12 hour and 40 min. soak test (CVS) which must be done in a laboratory.

In most cases an infrared HC/CO tester is not sensitive enough to measure the higher tailpipe emissions from a failing converter. Thus, a bad converter may allow enough emissions to escape so that the truck is no longer in compliance with Federal or state stands, but will still not cause the needle on a tester to move off zero.

The chemical reactions which occur inside a catalytic converter generate a great deal of heat. Most converter problems can be traced to fuel or ignition system problems which cause unusually high emissions. As a result of the increased intensity of the chemical reactions, the converter literally burns itself up.

As long as you avoid severe overheating and the use of leaded fuels it is reasonably safe to assume that the converter is working properly. If you are in doubt, take the truck to a diagnostic center that has a tester.

If the catalytic converter becomes blocked the engine will not run. The converter has 5 year or 50,000 mile warranty; contact your local Nissan dealer for more information.

REMOVAL & INSTALLATION

Removal and installation of the various exhaust system components including the catalytic converter assembly is found in
Engine & Engine Overhaul
.